The mechanisms by which the genetic and non-genetic factors which influence mammary tumor virus expression in mice have been examined. The glucocortoid hormone dexamethisone has been found to induce high levels of mammary tumor virus-specific RNA in the absence of C-type viral RNA in mammary tumor cell lines from mice. The mechanism by which the hormone induces this RNA has been investigated and has been found to be a primary effect of glucocorticoids not requiring new protein synthesis. An in vitro assay has been developed to study this which allows in vitro the detection of viral-specific RNA and the factors which regulate viral synthesis. Conclusive genetic data has been obtained that mouse mammary tumor virus expression is naturally genetically determined and is a primary cause of mammary tumors in mice.